Nanocarbon is a new material, represented by carbon nanotube, which draws attentions in various fields of energy, electronics, chemistry, pharmaceuticals, optics, materials and mechanics, etc. Carbon nanotubes include single- and multi-layered types and cup-stack types. The single- and multi-layered types are needle-like carbon molecules having a diameter in the order of nanometers and have a structure of graphene rolled into a cylinder. Those having a multi-layer structure composed of concentrically placed graphene cylinders are called multi-walled carbon nanotubes, while those consisting of a single layer of graphene cylinder are called single-walled carbon nanotubes.
Incidentally, there have conventionally been a number of procedures provided for refining crude carbon nanotube products. For example, Japanese Unexamined Patent Publication (Kokai) No. 2000-290008 discloses a process for refining carbon tubes which comprises the first step of dispersing a crude product containing carbon tubes into a gold colloidal solution, the second step of removing a solvent from the gold colloidal solution containing the crude product and the third step of heating the crude product under oxygen atmosphere. This technique utilizes the principle that gold tends to act as a catalyst when rendered particulate and oxidize carbon at a low temperature.
In the technique as described in Japanese Unexamined Patent Publication No. 2000-290008, an example of solvents for the solution to be used is water; however, the carbon nanotubes per se exist as dispersed in an aqueous solution without being dissolved.
In addition, the Preliminary Proceedings of 51st Symposium on Macromolecules, Oct. 2 to 4, 2002 at Kyushu Institute of Technology, Kitakyushu, Japan discloses a technique of solubilizing single-layered carbon nanotubes into water. This technique, however, solubilizes carbon nanotubes into water by surface-treating the carbon nanotubes with an amphipathic compound having a bilene group to impart hydrophilicity to their surfaces. Thus, when the surfaces are modified, such modifications must be removed afterward depending on the application, which makes it quite cumbersome.
The present invention aims to provide a novel technique for solubilizing nanocarbons into water without modifying nanocarbon surfaces.